Enable coveralls.io service.

Coveralls is a web service to help you track your code coverage
over time, and ensure that all your new code is fully covered.

Also add a status badge in README.

Change-Id: I6c2a914ce74e6af719056d0a4846465533988ba6
Remove --use-mirror from travis.yml

This option is deprecated.

Change-Id: I6b3aba6515c2b439def336ddcc54eacc53d8030c